1. Field of the Invention
The present invention relates to a device for the remote detection of radiation It in particular applies to dosimetry and more specifically to microdosimetry.
2. Description of the Prior Art
In order to perform a remote measurement of a radiation, such as a X or gamma radiation, it is known to use an optical fibre at one end of which is placed an element able to emit light in the presence of the radiation.
In this way the radiation is detected by the interaction thereof with the material constituting the elements traversed by the said radiation The lower the material quantity intervening in the detection, the lower the potential sensitivity of the thus obtained detector.
Thus, the energy deposited by the radiation is expressed in joules per kilogram of material. For a given emitter, the luminescence produced is proportional to the deposited energy.
For a crystal such as NaI, it is necessary to deposit there approximately 25 eV in order to produce a photon, whose wavelength is approximately 410 .mu.m.
Thus, for a given dose measurement, when the mass decreases, the corresponding energy decreases and the same applies with respect to the brightness and consequently the sensitivity.
The lowest detectable dose corresponds to the sensitivity threshold of the light detector (e.g. a photomultiplier). A photomultiplier having a 20% photocathode quantum efficiency will detect a minimum pulse signal of about 10 photons of 2 to 3 eV (reaching its entrance window).
The measured deposited energy is dependent on the minimum energy deposited by each particle and the number of particles converted in the detecting element. If the mass of the detector decreases, the detectable dose threshold increases.
An optical fibre has the advantage of being able to transmit light over considerable distances with low energy losses.
The use of an optical fibre imposes a certain number of constraints, which affect the sensitivity of the associated detector.
Moreover, the coupling of a material with an optical fibre introduces two limitations, namely a dimensional limitation, bearing in mind the diameter of the optical fibre core, and an angular limitation imposing that the collected light is accepted by the optical fibre bearing in mind its numerical aperture. Consequently the optical fibre limits the material quantity producing light accepted and transmitted by said fibre.
Thus, the light collected by the optical fibre is a function of said numerical aperture, the distance between the production point of said light and the corresponding end of the fibre and the distance from said point to the axis of said fibre
Thus, e.g. in the case of a silica optical fibre (optical index approximately 0.27) and a detecting crystal optically coupled to said fibre and whose dimensions, taken perpendicularly to the axis thereof, are large compared with those of the fibre, also taken perpendicular to said axis, the light produced by the interaction of radiation with the crystal is not accepted in the full angular aperture of the fibre unless it is produced in the immediate vicinity of the end thereof.
The light quantity accepted by the fibre decreases rapidly when the point at which said light is produced is beyond a distance of approximately 0.5 to 1 mm from the corresponding end of the fibre.
A known optical radiation detector is constituted by an inorganic scintillator optically coupled to a photomultiplier. In order to obtain such a detector, a large diameter block of said inorganic scintillator is placed in front of the photo-multiplier photocathode.
In this case, the problem of the collection of the light resulting from the interaction of the radiation with the scintillator is solved by placing said scintillator block in a box, whose inner face is coated with a high albedo diffusing or reflecting deposit (e.g. a MgO layer).
Thus, the collection of light causes no problems in the case of a detecting crystal coupled to a photomultiplier if the thickness of said crystal is roughly equal to or smaller than the transverse dimensions of said crystal, said transverse dimensions being approximately equal to the diameter of the photocathode of the photomultiplier.
However, the collection of the light produced in a detecting crystal by an optical fibre coupled to said crystal causes a problem, as stated hereinbefore, because the dosimetric measuring signal, which is the light signal transmitted by the fibre, decreases rapidly when the light is produced beyond a relatively small distance from the corresponding end of the fibre.
For the detection of radiation by means of optical fibres, it is known to use scintillating, plastic, optical fibres from the following document:
(1) EP-A107 532 corresponding to U.S. Pat. No. 4,552,431. PA1 (2) "Scintillating fibre detector system for spacecraft dosimetry", C. P. W. Boeder, L. Adams and R. Nickson, RADEC '93, Saint Malo, France, 13-16 September 1993. PA1 (3) "Fibre-optic nuclear detector system", technical description, production version 1993-FND-C2, SENSYS (Sensor System) P.O. Box 411 2200AK NOORDWIJK, The Netherlands. PA1 (4) U.S. Pat. No. 5,030,834 (Lindmayer et al).
A use of such scintillating optical fibres in dosimetry is described in the following document:
Reference can also be made in this connection to the following document:
Document (1) describes processes for producing scintillating optical fibres having a polystyrene core. These fibres are used in large particle detectors, generally for locating charged particles (such as electrons, protons and .pi. mesons) and also in e.sup.-, .gamma. power calorimeters having a high spatial resolution and a high energy resolution. These calorimeters are constituted by alternate layers of fibres and an absorbent element, which converts the incident radiation.
Documents (2) and (3) describe a high resolution dosimetry apparatus using approximately 10 cm of a scintillating optical fibre of the type referred to in document (1).
This polymeric material, scintillating optical fibre has a very low capacity for interaction and conversion of the energy of the .gamma. photons. It is therefore placed in a thin, aluminium metallic cylinder, which serves as the radiation converter.
The light emitted by the scintillating fibre under the effect of ionizing radiation is then transmitted by an undoped optical fibre, which also has a polystyrene core, to a GaAsP semi-conductor detector having a very low dark current.
Another dosimetry apparatus is mentioned in the following document:
This document relates to a dosimetric measuring apparatus, whereof the sensitive element is a doped strontium sulphide block having luminescence properties. It is a question of optically stimulated luminescence, This block, which has a side length of a few millimeters, is placed at the end of a silica optical fibre with a diameter of 0.2 mm.
The doped material used contains elements such as samarium or curopium, which have metastable levels. These storage levels, under the effect of the radiation, are stored in proportion to the dose received.
The light emission of a YAG laser, supplied to the SrS block by means of the optical fibre, by emptying said storage levels makes it possible to stimulate a light emission, which is proportional to the dose received.
In documents (2) and (3) it is a question of a plastic optical fibre guided structure. However, plastic materials have mediocre properties for detecting gamma radiation. It is generally preferable for gamma radiation detection purposes to use inorganic crystals of higher atomic number Z such as NaI or BGO crystals.
In the case of document (4), the material used is able to detect gamma radiation. However, the effective volume, which produces the light accepted by the optical fibre, is very limited. This corresponds to the disadvantage referred to hereinbefore.
Thus, no matter what the size of the SrS crystal placed at the end of the optical fibre, the effective volume is limited, on the one hand due to the fact of the diameter of the optical fibre (200 .mu.m) and on the other in thickness, due to the limitation to the numerical aperture.
For example, for a fibre with a diameter of 200 .mu.m and an aperture of .+-.15.degree. (silica/silicone fibre), the collected light decreases rapidly for distances to the fibre entrance face exceeding 0.8 mm.
Dosimetric detectors are also known which comprise optical fibres, but which do not use luminescence phenomena. These dosimetric detectors use the absorption induced in the optical fibres by structural modifications created by the radiation.
Such detectors are less sensitive, require greater fibre lengths and operate in accordance with an accumulation principle, which makes it necessary to frequently replace the optical fibres. Moreover, limited heat recovery effects, optionally at ambient temperature, disturb the accuracy of the measurements.